Faulty GPS or Faulty Race Distance? (Read 882 times)

Has anyone ever ran a race where your GPS was significant different than the race distance? I just finished a full marathon in Canada. It was a small race, less than 200 people, so other than the km markers along the way, there were no clocks or anything. I figured that since I had a GPS that I wouldn't need to worry about learning my pace in kms. My longest race with my GPS so far had been a 10k, but it was exactly on the mark. Anyway, at 26 miles, thinking I was in the home stretch and trying desperately to finish in under 4 hrs, the finish line was no where in sight. When I finally crossed the finish line my GPS had logged 26.5 miles, and the time was 4:01. I was very dissapointed. Can a GPS be that off, or is there a chance that the race distance was wrong?

That's about right. For the last few marathons that I ran with the GPS it measured between 0.2 and 0.25 miles long. I always assume I'm going about 5 seconds/mile slower than what the watch says. Still, it's better than relying on mile markers which either sometimes don't exist, or you miss them...

What they said. On my half marathon my GPS said 13.23 miles which was very close I thought, but extend that out to a marathon distance and it would have shown 26.46 miles instead of 26.2 miles which is pretty close to what yours showed.

Courses with a lot of twists and turns are going to be off farther than courses that are straight typically as well. Part of it is going to be that the course is measured as if you made the perfect angle on every turn, and part of it is that you don't run in a perfectly straight line, and part of it is going to be GPS error.

The worst thing that happened to me was when I went under a long underpass at the start of a race and my GPS logged a big sideways hop and added about 1/4 mile to my distance. My splits were messed up for the rest of the race.

There are many variables and this is normal. Best bet is to adjust to the road markers.

Actually I think the answer to her last question is both, not neither. A GPS can be that off and there is a chance that the race distance was wrong.

Unless there are other reasons to believe the course was long, a final gps reading of 26.5 miles for a marathon race would NOT cause me to think the course was long. I'd assume it to be about the correct distance.

In my marathon 2 days ago, my garmin was consistently off by about 5 seconds against each mile markers. My gps recorded about 26.4 miles. So when I read 9:00/mi on my garmin, I was actually running something like 9:05/mi. In addition to GPS inaccuracy, I also attribute that to my not running the race line and sticking to the middle of the road most of the time.

All variations listed here are within standard error of a consumer GPS unit. Also note that all listed errors were measuring long, which again is consistent with the typical behavior of a GPS unit with unimpeded satellite signal (if it has intermittent signal it is more likely to measure short, especially on curvy courses).

However, you have no data to know whether the course was accurate or not. The GPS unit won't tell you that unless, say, you measured it 5 times and it was 30 miles +/- 0.25 miles each time.

"If you want to be a bad a$s, then do what a bad a$s does. There's your pep talk for today. Go Run." -- Slo_Hand

As said about, it could be GPS, it could be course measurement, it could be both, or it could be neither. You measured about 1% long with your GPS. Not that much discrepancy when you think about it. Anecdotally, we see a lot of cases where the GPS distance is longer than the specified course because the runner wasn't able to take the exact tangents on turns and road crossings, and thus did not run the shortest possible distance.

Thanks everyone, guess I should have researched this a little before hand, that way I probably would have still used my GPS but set it in metric so I could compare against the markers along the way....but instead I'm simply left disappointed about not being 51 seconds faster to get under 4 hrs.